The invention relates to a method of using recycled plastic material, in which method crosslinked polyethylene is mixed with the base material, e.g. polyolefin, of the product to be produced, in such a way that the proportion of the recycled crosslinked polyethylene in the mixture is less than 30%, and the mixture is supplied through an extruder.
The invention also relates to apparatus for using recycled plastic material, the apparatus comprising a grinder for pre-processing the recycled material and an extruder for making an extruded product from the mixture of base material, such as polyolefin, and crosslinked polyethylene.
The invention also relates to a plastic product made by the extruder, the product primarily comprising base material, such as polyolefin, and less than 30% of recycled crosslinked polyethylene.
Recycling of plastics waste is often rather difficult, and the use of recycled crosslinked polyethylene for making plastic products is particularly difficult, since crosslinked polyethylene cannot be re-melted by standard methods. German Offenlegungsschrift 19 503 519 discloses a method in which a maximum of 25% by weight of crosslinked polyethylene is mixed with other plastic material and supplied to an extruder, in which the mixture is subjected to an elevated temperature of above 400xc2x0 C., after which the mixture is mixed in a mixer and a plastic product is produced by the extruder. The method concerned is complicated and requires apparatus in which the temperature of the mixture can be raised very high, whereby the method and apparatus become complicated and expensive to implement.
The object of the invention is to provide a method and apparatus by which crosslinked polyethylene can be re-used in a fairly cost-effective and simple manner and a very good product can be produced from the recycled crosslinked polyethylene.
The method of the invention is characterized by grinding the crosslinked polyethylene to powder form before it is supplied to the extruder.
Further, the apparatus of the invention is characterized in that the grinder comprises at least one stator and at least one rotatable rotor whose surfaces placed against each other are arranged to define a conical feed gap between them, the rotor and the stator having grooves for grinding the grindable material and for discharging it from the grinder.
The plastic product according to the invention is also characterized in that the crosslinked polyethylene is powder that is mixed with the base material.
The essential idea of the invention is that the crosslinked polyethylene is ground to powder form and mixed with polyolefin so that the proportion of crosslinked polyethylene in the mixture is less than 30% by weight and that the mixture is supplied through the extruder to produce an extruded plastic product. The idea of a preferred embodiment is that the crosslinked polyethylene is ground by grating and tearing so that the powder orientates. The idea of another preferred embodiment is that a conical grater is used for the grinding.
The advantage of the invention is that it enables cost-effective and simple recycling of crosslinked polyethylene, while providing a product that contains recycled crosslinked polyethylene. By grinding the crosslinked polyethylene at a low temperature by grating and tearing, such ground crosslinked polyethylene is obtained that is contained in a product whose tensile strength and elastic modulus are, surprisingly, very good. The use of standard filling agents typically improves the elastic modulus. Instead, the improvement in the tensile strength is particularly surprising and may be caused by the fact that the grating and tearing of the material at a low temperature in fact orientate a crosslinked particle so that its strength continues to grow to some extent and will not disappear completely during the extrusion step that follows. The use of a conical grater for the grinding is advantageous, for example, because the clearance is easy to adjust in it, and this has a decisive influence on the capacity and particle size distribution. The essential feature, however, is that the grindable pieces can be compressed to a high pressure in the conical or partly conical grinder, while the shape of the cross-section of the groove in the screw is maintained unchanged. The heat due to friction generated in this way can be reduced and the consumption of energy thereby decreased.